Potentiometric Determination of Stability Constants of Binary Vanillic Acid Complexes with Cu(II), Co(II), Fe(II), Ni(II), Zn(II), and Cd(II) Ions in 1:2 Metal–Ligand Systems

The present study investigates the binary complexation behaviour of vanillic acid (4-hydroxy-3-methoxybenzoic acid) with Cu(II), Co(II), Fe(II), Ni(II), Zn(II) and Cd(II) ions using Irving-Rossotti potentiometric pH-titration technique. Potentiometric titrations were carried out at constant ionic strength and at 27 ± 0.5 °C for a metal: ligand ratio of 1:2. The proton-ligand and metal-ligand stability constants were determined by the Calvin-Bjerrum method as modified by Irving and Rossotti. The proton-ligand stability constant of vanillic acid was determined by both half-integral and pointwise methods, yielding pK values of 2.95 and 2.97 respectively, which showed excellent agreement and confirmed the reliability of the experimental procedure. The shift of the acid–ligand–metal titration curves relative to the ligand curve confirmed the formation of stable coordination complexes. The calculated stability constants showed that the most stable complex was Cu(II) (log K = 4.541), followed by Co(II), Zn(II), Cd(II), Fe(II) and Ni(II). The ionic radius, charge density, ligand field stabilization energy and electronic configuration on complex formation all influence the stability trend observed. The results confirm that vanillic acid is an efficient oxygen-donor ligand having the ability to form stable 1:2 binary complexes with transition metal ions. The results contribute to the understanding of solution equilibria and may be useful in coordination chemistry, bioinorganic chemistry and metal ion sequestration studies.